Method and apparatus for preparing low-concentration polysilicate microgels

ABSTRACT

A method and apparatus for preparing low-concentration polysilicate microgels from a water soluble silicate and a strong acid in which the silicate and acid are mixed at a rate to produce a Reynolds number of at least 4000, the mixture is aged and then diluted to a silica concentration of not more than 1.0 wt. %.

This is a division of application Ser. No. 07/887,793, filed May 26,1992, now U.S. Pat. No. 5,279,807.

BACKGROUND OF THE INVENTION

The present invention relates to an improved method and apparatus forpreparing low-concentration polysilicate microgels, i.e., aqueoussolutions having an active silica concentration of generally less thanabout 1.0 wt. %, which are formed by the partial gelation of an alkalimetal silicate or a polysilicate, such as sodium polysilicate, having inits most common form one part Na₂ O to 3.3 parts SiO₂ by weight. Themicrogels, which are referred to as "active" silica in contrast tocommercial colloidal silica, comprise solutions of from 1 to 2 nmdiameter silica particles which have a surface area of at least about1000 m² /g. The particles are linked together during preparation, i.e.,during partial gelation, to form aggregates which are arranged intothree-dimensional networks and chains. A critical aspect of theinvention is the ability to produce the microgels within a reasonabletime period, i.e., not longer than about 15 minutes until the microgelis ready for use, without the risk of solidification and with minimumformation of undesirable silica deposits within the processingequipment. Polysilicate microgels produced according to the inventionare particularly useful in combinations with water soluble cationicpolymers as a drainage and retention aid in papermaking.

The present invention is an improved method and apparatus forcontinuously preparing a low-concentration polysilicate microgel whichcomprises:

(a) simultaneously introducing a first stream comprising a water solublesilicate solution and a second stream comprising a strong acid having apKa less than 6 into a mixing zone where the streams converge at anangle of not less than 30 deg and at a rate sufficient to produce aReynolds number of at least about 4000 and a resulting silicate/acidmixture having a silica concentration in the range of from about 1.0 to6.0 wt. % and a pH in the range of from 2 to 10.5;

(b) aging the silicate/acid mixture for a period of time sufficient toachieve a desired level of partial gelation, usually for at least 10seconds but not more than about 15 minutes; and

(c) diluting the aged mixture to a silica concentration of not greaterthan about 1.0 wt. % whereby gelation is stabilized.

For best results, the silica concentration of the water soluble silicatestarting solution is in the range of from 2 to 10 wt. % silica, and theconcentration of the strong acid (e.g., sulfuric acid) is in the rangeof from 1 to 20 wt. % acid as the two streams are being introduced intothe mixing zone. The preferred conditions in the mixing zone are aReynolds number greater than 6000, a silica concentration in the rangeof 1.5 to 3.5 wt. % and a pH in the range of 7 to 10. The most preferredconditions are a Reynolds number greater than 6000, silica concentrationof 2 wt. % and a pH of 9.

The apparatus according to the invention comprises:

(a) a first reservoir for containing a water soluble silicate solution;

(b) a second reservoir for containing a strong acid having a pKa of lessthan 6;

(c) a mixing device having a first inlet which communicates with saidfirst reservoir, a second inlet arranged at an angle of at least 30 degwith respect to said first inlet which communicates with said secondreservoir, and an exit;

(d) a first pumping means located between said first reservoir and saidmixing device for pumping a stream of silicate solution from said firstreservoir into said first inlet, and first control means for controllingthe concentration of silica in said silicate solution while saidsolution is being pumped such that the silica concentration in the exitsolution from the mixing device is in the range of 1 to 6 wt. %;

(e) a second pumping means located between said second reservoir andsaid mixing device for pumping a stream of acid from said secondreservoir into said second inlet at a rate relative to the rate of saidfirst pumping means sufficient to produce a Reynolds number within saidmixing device of at least 4000 in the region where the streams convergewhereby said silicate and said acid are thoroughly mixed;

(f) mixture control means located within said exit and responsive to theflow rate of said acid into said mixing device for controlling the pH ofthe silicate/acid mixture in the range of from 2 to 10.5;

(g) a receiving tank;

(h) an elongated transfer loop which communicates with the exit of saidmixing device and said receiving tank for transferring said mixturetherebetween; and

(i) a dilution means for diluting the silicate/acid mixture in thereceiving tank to a silica concentration of not more than 1.0 wt. %.

In an alternate embodiment, the apparatus of the invention includes aNaOH reservoir and means for periodically flushing the production systemwith warm NaOH which has been heated to a temperature of from 40° to 60°C. whereby deposits of silica can be solubilized and removed.

In a further embodiment of the invention, an agitating gas stream suchas a stream of air or nitrogen or other inert gas can be introduced intothe mixing device described by means of an additional inlet located ator near the mixing junction. Gas agitation provides an importantindustrial benefit in that it permits low silicate flow rates to beemployed while maintaining the required turbulence and Reynolds numberin the mixing zone.

The method and apparatus of the invention are capable of producingstable polysilicate microgels within a convenient time frame of not morethan about 15-16 minutes, but usually within 30 to 90 seconds, withoutthe risk of solidification and with minimum formation of undesirablesilica deposits within the processing equipment. Temperature ofoperation is usually within the range of 0°-50° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the process of the invention whichincludes a NaOH reservoir and means for periodically flushing theproduction system.

FIG. 2 is a schematic diagram of a dual line polysilicate microgelproduction system which provides for uninterrupted microgel production.

DETAILED DESCRIPTION OF THE INVENTION

Active silica is a specific form of microparticulate silica comprisingvery small 1-2 nm diameter particles which are linked together in chainsor networks to form three-dimensional structures known as "microgels".The surface area of the active silica microparticulates, i.e., themicrogels, is at least about 1000 m² /g. General methods for preparingpolysilicate microgels are described in U.S. Pat. No. 4,954,220, theteachings of which are incorporated herein by reference. Of the methodsdescribed therein, the acidification of a dilute aqueous solution of analkali metal silicate with an inorganic acid or organic acid, i.e., astrong acid having a pKa of less than 6, is the method to which thisinvention is particularly applicable. The present invention provides forthe reliable and continuous preparation of low-concentrationpolysilicate microgels at the site of intended consumption withoutformation of undesirable silica deposits within the processing equipmentand at very reasonable aging times generally less than 15 minutes, andpreferably between from 10 to 90 seconds.

The method of the invention is carried out by simultaneously introducinga stream of a water soluble silicate solution and a stream of strongacid having a pKa less than 6 into a mixing zone or mixing junction suchthat the streams converge at an angle of generally not less than 30 deg.with respect to each other and at a rate which is sufficient to producea Reynolds number in the region where the two streams converge of atleast 4000, and preferably in the range of about 6000 and above.Reynolds number is a dimensionless number used in engineering todescribe liquid flow conditions within a tube or pipe. Numbers below2000 represent laminar flow (poor mixing environment) and numbers of4000 and above represent turbulent flow (good mixing environment). As ageneral rule, the larger the Reynolds number the better the mixing.Reynolds number, (Re), is determined from the equation ##EQU1## WhereQ=Flow in cubic feet per second

d=Density in pounds per cubic foot

D=Pipe diameter in feet

u=Viscosity in pounds per foot second.

The concentrations of the converging silicate solution and the acidstreams are controlled so that the resulting silicate/acid mixture thusproduced has a silica concentration in the range of 1 to 6 wt. % and apH in the range of 2 to 10.5. More preferably the silica concentrationis in the range of 1.5 to 3.5 wt. % and the pH is in the range of 7 to10. The most preferred operating conditions are with a Reynolds numberlarger than 6000, a silica concentration of 2 wt. % and a pH of 9.

Aging is generally accomplished in from 10 up to about 90 seconds bypassing the silicate/acid mixture through an elongated transfer loop inroute to a finished product receiving tank in which the mixture isimmediately diluted and thereafter maintained at an active silicaconcentration of not greater than 1.0 wt. %. Partial gelation whichproduces the three-dimensional aggregate networks and chains of highsurface area active silica particles is achieved during aging. Dilutionof the silicate/acid mixture to low concentration operates to halt thegelation process and stabilize the microgel for subsequent consumption.

The method of the invention and an apparatus for carrying it out willnow be discussed in greater detail in reference to the drawings in whichFIG. 1 is a schematic diagram of the process in its simplest form. Thesizes, capacities and rates described herein can be varied over wideranges depending primarily on the quantities of polysilicate microgelrequired and the expected rate of consumption. The sizes and capacitiesdescribed in reference to the drawings relate to a system for producing,i.e., generating, polysilicate microgel on a generally continuous basisfor consumption as a drainage and retention aid in a papermaking processin which the consumption rate ranges from about 10 to 4000 lbs. microgelper hour.

There is shown in FIG. 1 a dilution water reservoir 10, an acidreservoir 12, and a silicate reservoir 14. The reservoirs, i.e., tanks,are conveniently made of polyethylene, with the water reservoir having acapacity of 500 gallons, the acid reservoir having a capacity of 100gallons, and the silicate reservoir having a capacity of 300 gallons.Other vessels shown in FIG. 1 are NaOH flush tank 16 and finishedproduct receiving tank 18. The NaOH flush tank is made of anon-corrosive material, such as, for example, 316 stainless steel; ithas a capacity of 20 gallons and is heated with an electrical resistancedrum heater wrapped around it (Cole-Palmer, 2000 watts, 115 volts). Thefinished product receiving tank has a capacity of 1000 gallons and ismade of polyethylene.

A critical element of the process is mixing junction 20 which defines amixing zone in which a stream of acid and a stream of water solublesilicate are introduced along individual paths which converge within themixing zone at an angle generally not less than 30 deg. A mixing "T" or"Y" junction is suitable for practicing the invention and may readily beconstructed from an appropriately sized 316 stainless steel "Swagelok"compression coupling fitted with stainless steel tubing. A "T" junctionis generally preferred.

The rates at which the two streams enter, i.e. are pumped into, themixing zone are selected to produce a Reynolds number therewith of atleast 4000 and preferably up to 6000 or higher which results inpractically instantaneous and thorough mixing of the acid and silicatesuch that the resulting mixture has a silica concentration in the rangeof from 1.5 to 3.5 wt. % and a pH of from 7 to 10. Any convenientcommercial source of water soluble silicate can be employed, such as,for example, "PQ (N)" sodium silicate (41 Baume, SiO2:Na2O=3.22:1 byweight, 28.7 wt. % SiO2) marketed by the PQ corporation. The commercialsilicate is maintained undiluted in reservoir 14, usually at aconcentration of 24 to 36 wt. % as supplied by the manufacturer, untilit is needed. It is supplied to the mixing junction 20 via suitabletubing 22 (316 SS, 1/4 inch OD) by means of a low flow rate gear ormicropump 24 (e.g., Micropump Corp., model 140, max. flow 1.7 gpm).Non-corrosive materials of construction, e.g., 316 stainless steel, arepreferred to avoid any risk of corrosion and subsequent contamination.The silicate supply line also includes flow control valve 26 (Whitey,316 SS, 1/4 inch needle), magnetic flow meter 28 (Fisher Porter, 316 SS,1/10 inch size) and check valve 86 (Whitey, 316 SS, 1/4 inch diameter)for controlling and monitoring the amount and direction of silicateflow. In operation, dilution water is introduced into the silicatesupply line 22 at a convenient location upstream of the silicate/acidmixing junction 20 so as to adjust the silica concentration to a valuein the range of from 2 to 10 wt. %. To insure complete mixing ofsilicate and water an in-line static mixer 32 (Cole-Palmer, 316 SS, 1/2inch tubing, 15 elements) is provided followed by a check valve 30(Whitey, 316 SS, 1/2 inch diameter). The dilution water is supplied vialine 34 (1/2 inch OD, 316 SS) by centrifugal pump 36 (Eastern Pump, 1HP, max. flow 54 gpm), and a rotameter 38 (Brooks, Brass Ball, 3.06 gpmmax.). Control valve 40 (Whitey, 316 SS, 1/2 inch NE needle) and checkvalve 42 (Whitey, 316 SS, 1/2 inch diameter) can be employed to thecontrol flow rate and direction.

Although a wide range of acidic materials, such as, for example, mineralacids, organic acids, acid salts and gases, ion-exchange resins and thesalts of strong acids with weak bases, have been described for use inpreparing active silica, the simplest and most convenient means ofacidification is with a strong acid having a pKa less than 6. Thepreferred acid is sulfuric acid. Commercial grades manufactured by DuPont and others are generally suitable. In operation, a stock solutionof acid is maintained at a concentration in the range of from 5 to 100wt. % in acid reservoir 12. The acid is pumped using a gear or similarmicropump 44 (e.g., Micropump model 040, 1/4 HP, max. flow 0.83 gpm) tojunction mixer 20 through line 46 (316 SS, 1/4 inch OD) and check valve88 (Whitey, 316 SS, 1/4 inch diameter). A single loop controller 90(Moore, Model 352E) is combined with pH transmitter 48 (Great LakesInstruments, Model 672P3FICON) and pH Probe 48A (Great LakesInstruments, Type 6028PO) to regulate the flow of acid to junction mixer20 via automatic flow control valve 50 (Research Controls, K Trim, 1/4inch OD, 316 SS) in response to the pH of the silicate/acid mixturemeasured at the exit of the junction mixer. An automatic three-way valve52 (Whitey, 316 SS, 1/2 inch diameter) is also employed within thecontrol system to allow for the possibility of having to divert off-specsilicate/acid mixture to the sewer. Dilution water from water reservoir10 is provided via line 54 (316 SS, 1/2 inch OD) to dilute the acidsupply upstream of junction mixer 20 to a predetermined concentration inthe range of from 1 to 20 wt. %. A static mixer 56 (Cole-Palmer, 316 SS,1/2 inch diameter, 15 turns) is provided downstream of the point wheredilution water is introduced into the acid supply line to insurecomplete mixing and dilution of the acid. A rotameter 58 (Brooks, BrassBall, 1.09 gpm. maximum), control valve 60 (Whitey, 316 SS, 1/2 inchneedle) and check valve 62 (Whitey, 316 SS, 1/2 inch diameter) are usedto control flow rate and flow direction of the dilution water.

The silicate/acid mixture which exits junction mixer 20 has preferably aSiO₂ concentration in the range of from 1.5 to 3.5 wt. % and a pH in therange of from 7 to 10. Most preferably the silica concentration ismaintained at 2 wt % and the pH at 9. The mixture is passed through anelongated transfer line 64 (11/2 inch schedule 40 PVC pipe, 75 feet inlength) in route to finished product receiving tank 18. The length ofthe transfer line is selected to insure that the transfer will take atleast 10 seconds, but preferably from about 30 seconds to 90 seconds,during which time "aging" or partial gelation of the mixture takesplace. Transfer time can be as long as 15-16 minutes at very low flowrates and still produce satisfactory results. Dilution water fromreservoir 10 is added via line 66 (316 SS, 1/2 inch od) to the mixturejust prior to its entry into finished product receiving tank 18 or atany other convenient location so long as the silicate/acid mixture isdiluted to an SiO₂ concentration of less than 1.0 wt.% which stabilizesthe gelation process. Dilution water is supplied with centrifugal pump68 (Eastern, 316 SS, 1 HP, 54 gpm maximum), and flow control isaccomplished at a predetermined rate with control valve 70 (Whitey, 316SS, 1/2 inch needle) and rotameter 72 (Brooks, SS Ball, 12.46 gpmmaximum). The finished product receiving tank 18 is provided with alevel control system 74 (Sensall, Model 502) which operates inconjunction with an automatic three-way valve 76 (Whitey, 316 SS, 1/2inch diameter) to divert flow of the silicate/acid mixture to the sewerif the level of finished product becomes too high.

After a period of continuous operation, which depends on the amount ofactive silica produced, it may be desirable to cease generation of theactive silica and flush the mixing junction 20 and that portion of thesystem which is downstream, i.e., piping, valves, transfer lines, etc.,which have been in contact with the silicate/acid mixture, with waterand warm NaOH. Flushing the system removes any undesirable silicadeposits which may have accumulated in parts of the apparatus where therequired turbulent flow conditions could not have been maintained due todesign restrictions, as for example in the region of pH measurement. Theflushing procedure helps maintain the system free of silica depositionand is begun by first shutting off dilution pump 68, acid pump 44 andsilicate pump 24. Dilution water from pump 36 is then circulated throughthe downstream portion of the system for about 5 minutes, after whichpump 36 is shut off, and the dilution water reservoir is isolated byclosing valves 40, 60 and 70. Three-way automatic valves 52 and 76, andmanufal valves 78, 80 and 82 (all Whitey, 316 SS, 1/2 inch OD) are thenactivated along with centrifugal circulating pump 84 (Eastern, 316 SS,1.5 HP, 15 gpm maximum) to allow NaOH, maintained at a concentration of20 wt. % and a temperature in the range of from 40° to 60° C., tocirculate through the downstream portion of the system for generally notlonger than about 20-30 minutes. The NaOH circulating pump 84 and theflush tank 16 are then isolated from the system by again activatingthree-way valves 80 and 82, and dilution water is again flushed throughthe downstream system and released to the sewer. Having completed thecleaning/flushing procedure, the production of active silica can beresumed.

Referring now to FIG. 2, there is shown a schematic diagram of a dualline production system for active silica, whereby one line can beoperational at all times while the other line is being flushed or beingmaintained in a stand-by condition. The component parts are numbered inaccordance with FIG. 1. A commercial system according to either of FIGS.1 or 2, will generally be constructed of stainless steel or polyvinylchloride tubing of generally one inch diameter or less, depending on therequirement for active silica. When stainless steel tubing is used,connections of the various instruments, fittings, valves, and sectionscan be conveniently made with "Swagelok" compression joints.

EXAMPLE 1 Demonstrating the effect of turbulence in reducing silicadeposition

A laboratory generator for producing polysilicate microgels wasconstructed according to the principles described in FIG. 1. Thesilicate and sulfuric acid feeds, before dilution and mixing, contained15 wt. % silica and 20 wt. % acid respectively. The critical junctionmixer was constructed from a 1/4 inch, 316 stainless steel "Swagelok"T-compression fitting fitted with 6 inch arms of 1/4 inch od 316 SStubing. The internal diameter of the fitting was 0.409 cm. For the testsin which a gas was introduced into the mixing junction a similar"Swagelok" X-compression coupling was used with the fourth arm of the Xas the gas inlet. An inline filter comprised of 1 inch diameter 60 meshstainless steel screen was placed about 12 inches from the acid/silicatejunction to trap particulate silica. The screen was weighed at thebeginning of each test and again at the end of each test, after washingand drying, so as to give a measure of silica deposition. All tests wererun so as to maintain conditions of 2 wt. % silica and pH 9 at the pointof silicate acidification and each test was run for sufficient time toproduce a total amount of 1,590 gms. of polysilicate microgel. Theresults of the tests are given in Table 1 below. Liquid flow representsthe total liquid flow, that is, the flow of the combined silicate/acidmixture in the exit tube. In the tests where a gas was introduced toenhance liquid flow and turbulence, the Reynolds number was calculatedon the basis of the increased flow rate of the liquid portion alone,assuming that liquid density and viscosity did not change appreciably.This method of calculation was adopted because there is no ready formulafor calculating the Reynolds number of liquid/gas mixtures.

                                      TABLE 1                                     __________________________________________________________________________    Silica Deposition As A Function Of Reynolds Number                            Test                                                                              Reynolds                                                                            Run Time                                                                            Liquid Flow                                                                           Gas Flow                                                                            Silica                                          No. No.   mins. ml/m    ml/m  deposited, gms.                                 __________________________________________________________________________    1   1,036 330   250     none  0.339                                           2   2,072 165   499     none  0.135                                           3   4,144  83   999     none  0.009                                           4   6,217  55   1,498   none  0.007                                           5   10,362                                                                               33   2,497   none  0.002                                           6   12,433                                                                               27   2,996   none  0.008                                           7   12,260                                                                              120   694     Air, 2,260                                                                          0.008                                           8   9,064 120   694     Air, 1,490                                                                          0.005                                           9   5,375 120   694     Air, 601                                                                            0.004                                           10  5,375 120   694     N2, 601                                                                             0.014                                           __________________________________________________________________________

A comparison of the results of Tests 1 and 2 with the results of Tests3-10 clearly demonstrate the beneficial effect of turbulent liquid flow(Reynolds number above 4,000) in reducing the amount of silicadeposition observed. Under turbulent flow conditions, the average silicadeposition of 0.007 gms. represented only 0.0004% of the total amount ofsilica processed.

EXAMPLE 2 Apparatus

A commercial sized apparatus for preparing active silica microgels wasassembled according to the schematic design shown in FIG. 1 andinstalled in a commercial paper mill. The apparatus, except for the rawmaterial supply reservoirs, was rigidly mounted on steel framework ontwo skids each measuring approximately six feet by eight feet. On skid 1was mounted inlets for connection to commercial supplies of sodiumsilicate and sulfuric acid and an inlet for city water which was usedfor dilution purposes. Also on skid 1 was mounted the dilution and flowcontrol means, the silicate/acid mixing junction, pH measurement and pHcontroller, sodium hydroxide flush reservoir, required pumps and valvesand the electrical controls. On skid 2 was mounted the ageing loop,finished product reservoir, level controller and required pumps andvalves. Overall height of each skid was about seven feet. Themanufacturers supply containers were used as reservoirs for the silicateand sulfuric acid and these were connected directly to the appropriateinlets on skid 1.

The apparatus was operated continuously for six days during which 0.5wt. % active silica was produced at a rate which varied between 3 and4.8 gallons per minute. At a production rate of 3 gpm, a Reynolds numberof 4250 was calculated for the mixing zone employed. No silicadeposition was observed within the junction mixer 20, although somesilica deposition was observed in the proximity of the pH probe locatedimmediately downstream from the junction mixer exit after 12 hours ofcontinuous operation. To alleviate this situation, a water/NaOH/waterflush sequence was conducted, which took less than 30 minutes, and thesystem was then returned to normal production. Over the entire six dayperiod, the apparatus operated without fault and produced active silicaof excellent quality which was utilized by the mill for the productionof a range of papers with different basis weights.

We claim:
 1. An apparatus for continuously producing a stable aqueouspolysilicate microgel which comprises:(a) a first reservoir containing awater soluble silicate solution; (b) a second reservoir containing anacid having a pKa of less than 6; (c) a mixing device having a firstinlet which communicates with said first reservoir, a second inletarranged at an angle of at least 30 degrees with respect to said firstinlet which communicates with said second reservoir, and an exit; (d) afirst pumping means located between said first reservoir and said mixingdevice for pumping a stream of silicate solution from said firstreservoir into said first inlet and, a first control means forcontrolling the concentration of silica within the range of from 1 to 6wt. % in the silicate/acid mixture in said mixing device while saidsolution is being pumped; (e) a second pumping means located betweensaid second reservoir and said mixing device for pumping a stream ofacid from said second reservoir into said second inlet at a raterelative to the rate of said first pumping means sufficient to produce aReynolds number within said mixing device of at least 4000 in the regionwhere said streams converge whereby said silicate and said acid arethoroughly mixed; (f) a mixture control means located within said exitand responsive to the flow rate of said acid into said mixing device forcontrolling the pH of the silicate/acid mixture in the range of from 2to 10.5; (g) a receiving tank, (h) an elongated transfer loop whichcommunicates with the exit of said mixing device and said receiving tankfor transferring said mixture therebetween; and (i) a dilution means fordiluting the silicate/acid mixture in the receiving tank to a silicaconcentration of not more than 1.0 wt. %.
 2. The apparatus of claim 3 inwhich said first inlet and said second inlet are arranged at a 90degrees angle with respect to each other and said acid is sulfuric acid.